I am having almost the exact same issue with my onboard Intel chipset. I
believe it is defaulting to some strange 6-channel digital output and it is
unclear to me how to switch it back to good old 2-channel analog. My MB manual
has detailed pictures of how to switch modes, but only using the supplied
Windows driver.
My latest idea is to try and bypass ALSA and see if OSS will work. I have only
been playing with Gentoo for a couple of months now, and on three computers I
have only ever used ALSA so I don't know if this will do it.
Basically, I unloaded ALSA, and then modprobed "sound" and "i810_audio" which
were previously compiled kernel (2.4.26-gentoo-r9) modules. The encouraging
thing about this is these two messages in /var/log/messages :
"Audio Controller supports 6 channels"
and
"Defaulting to base 2 channel mode"
I don't know yet if this did it, since I tried it over an SSH session while on a
break at work, so I have to wait until I get home.
Let me know if that works for you, & good luck.
-Steve